Click on a topic like Welcome to the Chalk Loft then look for the box at the top of the older posts that says New Post. That's for starting new topics. Click on that and a new big box will open for your postings. It sometimes opens slowly so wait for it. Also, you can open a previous post, read it and then click the Answer box at the top of the old post in order to leave your own response to that older post.
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ps. I forgot that when reading an older post you can also just scroll to the bottom of the page and answer in the big white response box. You don't have to click on the Answer box at the top of the old post.